Who would nowadays think of consulting a philosopher about a matter of life and death? Intellectuals in general have lost much of their prestige, but our own admiration for the philosopher-kings of this dwindling caste has evaporated. Just why this has happened was illuminated last month in a postmodern take on the old fairy-tale about the emperor's new clothes.

On to the stage came Bernard-Henri Lévy, the very model of a modern major generalist. Debonair and lightly tanned as ever, the great French public intellectual-at-large was touring the TV studios to promote his latest book, De la Guerre en Philosophie (roughly, "Philosophy Wars"). Whatever BHL — or "Bernadette", as he is irreverently known to those not captivated by his charisma — does, he does in style.

This time, he was attempting to cut down to size no mere contemporary, but one of the greatest philosophers of all time: Immanuel Kant. For centuries, the author of the Critique of Pure Reason had been revered as the seminal thinker of the Enlightenment. According to Lévy, however, it was time to acknowledge the truth: that Kant was a fraud, an "abstract fake, a pure spirit of pure appearance". As the source of German idealism, Kant was responsible for Hegel and Marx and ultimately for the failure of European intellectuals to resist totalitarianism. Lévy based his debunking of the creator of the Categorical Imperative on an obscure 20th-century philosopher, Jean-Baptiste Botul, who had launched a devastating critique of Kant in a series of hitherto little-known lectures to the "neo-Kantians of Paraguay".

The nature of philosophy is to assume that things are not what they seem. Here, a philosophical debate turned into a cautionary tale about the deceptiveness of appearances so delicious and yet so serious that it causes us to reconsider the impact we allow public intellectuals to have on our contemporary culture.

For the voice Lévy quoted as authoritative was an invention. The journalist Frédéric Pagès of the satirical magazine Le Canard enchaîné had penned a number of tongue-in-cheek pieces under the nom de plume Botul. Among his articles was La Vie Sexuelle d'Emmanuel Kant. That title alone ought to have been a clue that Botul was not quite what he seemed. As BHL must have known, Kant was a plain, pigeon-chested professor who remained a lifelong bachelor and never travelled more than a few miles from Königsberg (now Kaliningrad), where he was born. Sex played little part in his life and even less in his work.